Small-particle lithium iron phosphate positive electrode material and preparation method thereof, lithium ion battery and electric equipment

Abstract

The invention provides a small-particle lithium iron phosphate positive electrode material and a preparation method thereof, a lithium ion battery and electric equipment, and relates to the field of lithium ion batteries. Mixing iron phosphate, a lithium source, a carbon source and a solvent to obtain a mixture, and performing spray drying and tabletting treatment on the mixture to obtain a mixture raw sheet; and under inert gas, arranging the mixture raw sheet between graphite sheets, and electrifying for Joule heat treatment to obtain the small-particle lithium iron phosphate positive electrode material. By adopting the Joule heat heating method, the preparation time of the lithium iron phosphate material is shortened, the tendency of melting and growing between particles is reduced, the particles are independent, and thus the preparation of the small-particle lithium iron phosphate material is realized.

Description

Technical Field

[0001] The present application relates to the field of lithium-ion batteries, and particularly to a small-particle lithium iron phosphate cathode material, a preparation method thereof, a lithium-ion battery, and an electrical device. Background Art

[0002] With the development of new energy technologies, lithium iron phosphate (LiFePO4) has become a widely used battery material due to its excellent safety, environmental friendliness, and long cycle life. However, at low temperatures, conventional lithium iron phosphate materials have problems such as rapid capacity decay and poor capacity performance. This is mainly due to factors such as the slow diffusion rate of lithium ions and large ionic polarization under low-temperature conditions. Preparing lithium iron phosphate materials with small particle sizes can shorten the diffusion path of lithium ions and effectively improve the low-temperature performance of lithium iron phosphate materials.

[0003] The existing technologies in the industry for preparing small-particle lithium iron phosphate mainly include the following: 1) Controlling the heating rate: By controlling the heating rate, the particle size can be indirectly affected, and a slower heating rate is beneficial for forming smaller particles; 2) Biphasic synthesis method: A middle phase can be pre-synthesized at a lower temperature first, and then the final synthesis is carried out at a higher temperature. This method helps to better control particle growth; 3) Post-treatment ball milling: The synthesized lithium iron phosphate can be further mechanically ball milled to reduce the particle size. Selecting appropriate grinding media and optimizing the grinding time are the keys; 4) Selecting a small-particle-size iron phosphate precursor. The smaller the particle size of the precursor, the smaller the particle size of the resulting lithium iron phosphate product.

[0004] The existing technical solutions for preparing lithium iron phosphate involve mixing iron phosphate with a lithium source and a carbon source, and completing the preparation of the lithium iron phosphate material through a heating process in a reducing atmosphere. The heating process generally requires more than twenty hours. During this process, thermal diffusion and melting processes occur between lithium iron phosphate / iron phosphate particles, resulting in particle growth. Limited by the existing technology, it is difficult to prepare small-particle lithium iron phosphate materials by the solid-phase method.

[0005] Therefore, there is an urgent need to provide a method for preparing a small-particle lithium iron phosphate cathode material to solve the above problems. Summary of the Invention

[0006] The purpose of the present application is to provide a small-particle lithium iron phosphate cathode material, a preparation method thereof, a lithium-ion battery, and an electrical device to solve the above problems.

[0007] To achieve the above purpose, the first aspect of the present application provides a preparation method for a small-particle lithium iron phosphate cathode material, including:

[0008] Mixing iron phosphate, a lithium source, a carbon source, and a solvent to obtain a mixture, and performing spray drying and tabletting on the mixture to obtain a mixture original tablet;

[0009] Under an inert gas, place the original mixture sheet between graphite sheets, apply current for Joule heat treatment to obtain a lithium iron phosphate cathode material in small particles.

[0010] Optionally, the preparation method of the lithium iron phosphate cathode material in small particles satisfies at least one of the following conditions:

[0011] A. The lithium source includes lithium carbonate and / or lithium hydroxide;

[0012] B. The carbon source includes glucose and / or sucrose;

[0013] C. The solvent includes water;

[0014] D. The mass ratio of the iron phosphate, the lithium source, the carbon source and the solvent is 1:(0.4 - 0.6):(0.1 - 0.3):(1.5 - 2.0).

[0015] Optionally, before the spray drying, the mixture is also ground.

[0016] Optionally, the preparation method of the lithium iron phosphate cathode material in small particles satisfies at least one of the following conditions:

[0017] A. The grinding time is 0.5 h - 2 h;

[0018] B. The particle size of the zirconia beads for grinding is 0.01 mm - 0.2 mm.

[0019] Optionally, the preparation method of the lithium iron phosphate cathode material in small particles satisfies at least one of the following conditions:

[0020] A. The pressure for the tablet pressing treatment is 300 MPa - 800 MPa;

[0021] B. The diameter of the original mixture sheet is 20 mm - 40 mm;

[0022] C. The thickness of the original mixture sheet is 5 mm - 10 mm.

[0023] Optionally, the applied current for the Joule heat treatment is 1000 A·h - 3000 A·h, and the time is 10 s - 30 s.

[0024] The second aspect of the present application provides a lithium iron phosphate cathode material in small particles, which is prepared by the preparation method of the lithium iron phosphate cathode material in small particles.

[0025] Optionally, the primary particle size D50 of the lithium iron phosphate cathode material in small particles is less than or equal to 200 nm.

[0026] The third aspect of the present application provides a lithium-ion battery, including the small-particle lithium iron phosphate cathode material described above.

[0027] The fourth aspect of the present application provides an electrical device, including the lithium-ion battery described above.

[0028] Compared with the prior art, the beneficial effects of the present application include:

[0029] The preparation method of the small-particle lithium iron phosphate cathode material provided by the present application shortens the preparation time of the lithium iron phosphate material by using the Joule heating method, reduces the tendency of melting and growth between particles, makes the particles independent of each other, and thus realizes the preparation of the small-particle lithium iron phosphate material.

[0030] The small-particle lithium iron phosphate cathode material provided by the present application has a high diffusion rate of lithium ions and small ion polarization under low-temperature conditions.

[0031] The small-particle lithium-ion battery and electrical device provided by the present application have a long cycle life and slow capacity decay. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application and should not be regarded as limiting the scope of the present application.

[0033] Figure 1 SEM diagram of the small-particle lithium iron phosphate cathode material provided for Embodiment 1;

[0034] Figure 2 SEM diagram of the lithium iron phosphate cathode material provided for Comparative Example 1. Detailed Description of the Embodiments

[0035] As used herein, the terms:

[0036] "Prepared from" is synonymous with "comprising". As used herein, the terms "comprising", "including", "having", "containing" or any other variation thereof are intended to cover non-exclusive inclusion. For example, a composition, step, method, article or device containing the listed elements need not be limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article or device.

[0037] The conjunctive "consisting of" excludes any unrecited element, step, or component. If used in a claim, this phrase will render the claim closed, excluding materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause within the body of a claim rather than immediately following the subject, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0038] When an equivalent, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper preferred values and lower preferred values, this should be understood to specifically disclose all ranges formed by any pairing of any upper range limit or preferred value with any lower range limit or preferred value, whether or not that range is separately disclosed. For example, when the range "1 to 5" is disclosed, the described range should be interpreted to include the ranges "1 to 4", "1 to 3", "1 to 2", "1 to 2 and 4 to 5", "1 to 3 and 5", etc. When a numerical range is described herein, unless otherwise stated, the range is intended to include its end values and all integers and fractions within that range.

[0039] In these embodiments, unless otherwise specified, the parts and percentages are by mass.

[0040] "Parts by mass" refers to the basic measurement unit representing the mass ratio relationship of multiple components. 1 part can represent any unit mass, such as 1 g or 2.689 g, etc. If we say that the mass part of component A is a parts and the mass part of component B is b parts, it means the mass ratio of component A to component B is a:b. Or, it means the mass of component A is aK and the mass of component B is bK (K is any number representing a multiple factor). It should not be misunderstood that, different from the number of mass parts, the sum of the mass parts of all components is not limited to 100 parts.

[0041] "And / or" is used to indicate that either or both of the stated circumstances may occur. For example, A and / or B includes (A and B) and (A or B).

[0042] The first aspect of the present application provides a method for preparing a small-particle lithium iron phosphate cathode material, comprising:

[0043] Mixing iron phosphate, a lithium source, a carbon source, and a solvent to obtain a mixture, and subjecting the mixture to spray drying and tabletting to obtain a mixture original tablet;

[0044] Under an inert gas, placing the mixture original tablet between graphite sheets and conducting Joule heat treatment by electrification to obtain a small-particle lithium iron phosphate cathode material.

[0045] It should be noted that Joule heat is the process of converting electrical energy into internal energy (heat energy), and the heating principle of Joule heat is as follows:

[0046] According to the equation Q = I 2 Rt;

[0047] where Q is the calorific value, I is the working current, R is the resistance of the graphite sheet, and t is the working time. When an electric current passes through a conductor, the conductor will generate heat and provide heat energy to the mixture original sheet for heating.

[0048] The chemical reaction process that occurs during the heating of the mixed original sheet is as follows (taking glucose and lithium carbonate as examples):

[0049] Carbon source decomposition: C6H 12 O6H2O (glucose) → 6C + H2O↑;

[0050] LFP synthesis: FePO4 + 0.5LiCO3 + C → LiFePO4 / C + CO2↑;

[0051] Carbon source decomposition: The carbon source undergoes thermal decomposition to release water vapor and generate amorphous carbon. Subsequently, the amorphous carbon undergoes a conversion reaction with iron phosphate and lithium carbonate; The role of amorphous carbon has two points: 1. Reducing agent: Carbon reduces Fe 3+ (in FePO4) to Fe 2+ (in LiFePO4); 2. Conductive agent: The remaining carbon forms a coating layer to improve the electronic conductivity of the lithium iron phosphate material.

[0052] The Joule heat heating process is very rapid, so the iron phosphate precursor particles will not adhere, agglomerate, or grow, and can rapidly react in-situ and be converted into lithium iron phosphate in one step. In some embodiments, the preparation method of the small particle lithium iron phosphate cathode material satisfies at least one of the following conditions:

[0053] A. The lithium source includes lithium carbonate and / or lithium hydroxide;

[0054] B. The carbon source includes glucose and / or sucrose;

[0055] C. The solvent includes water;

[0056] D. The mass ratio of the iron phosphate, the lithium source, the carbon source, and the solvent is 1:(0.4 - 0.6):(0.1 - 0.3):(1.5 - 2.0).

[0057] Optionally, the mass ratio of iron phosphate, lithium source, carbon source and solvent can be any value between 1:0.4:0.1:1.5, 1:0.5:0.1:1.5, 1:0.6:0.1:1.5, 1:0.4:0.2:1.5, 1:0.4:0.3:1.5, 1:0.4:0.1:1.6, 1:0.4:0.1:1.7, 1:0.4:0.1:1.8, 1:0.4:0.1:1.9, 1:0.4:0.1:2 or 1:(0.4 - 0.6):(0.1 - 0.3):(1.5 - 2.0).

[0058] In some embodiments, before the spray drying, the mixture is also ground.

[0059] It should be noted that grinding can mix the materials more evenly, break the iron phosphate particles, reduce the particle size of the iron phosphate particles, and obtain raw materials with small particle sizes.

[0060] In some embodiments, the method for preparing the small-particle lithium iron phosphate cathode material satisfies at least one of the following conditions:

[0061] A. The grinding time is 0.5 h - 2 h;

[0062] Optionally, the grinding time can be 0.5 h, 1 h, 1.5 h, 2 h or any value between 0.5 h - 2 h;

[0063] B. The particle size of the zirconia beads for grinding is 0.01 mm - 0.2 mm.

[0064] Optionally, the particle size of the zirconia beads for grinding can be 0.01 mm, 0.1 mm, 0.2 mm or any value between 0.01 mm - 0.2 mm.

[0065] In some embodiments, the method for preparing the small-particle lithium iron phosphate cathode material satisfies at least one of the following conditions:

[0066] A. The pressure for the tablet pressing treatment is 300 MPa - 800 MPa;

[0067] Optionally, the pressure for the tablet pressing treatment can be 300 MPa, 400 MPa, 500 MPa, 600 MPa, 700 MPa, 800 MPa or any value between 300 MPa - 800 MPa;

[0068] B. The diameter of the original mixture tablet is 20 mm - 40 mm;

[0069] Optionally, the diameter of the original mixture tablet can be 20 mm, 25 mm, 30 mm, 35 mm, 40 mm or any value between 20 mm - 40 mm;

[0070] C. The thickness of the original mixture sheet is 5 mm - 10 mm.

[0071] Optionally, the thickness of the original mixture sheet can be 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, or any value between 5 mm - 10 mm.

[0072] It should be noted that the shape of the original mixture sheet includes, but is not limited to, regular shapes such as circular, square, rhombus, triangle, trapezoid, or irregular shapes.

[0073] In some embodiments, the applied current for the Joule heat treatment is 1000 Ah - 3000 Ah, and the time is 10 s - 30 s.

[0074] Optionally, the applied current for the Joule heat treatment can be 1000 Ah, 1500 Ah, 2000 Ah, 2500 Ah, 3000 Ah, or any value between 1000 Ah - 3000 Ah, and the time can be 10 s, 15 s, 20 s, 25 s, 30 s, or any value between 10 s - 30 s.

[0075] It should be noted that when the applied current for the Joule heat treatment is 1000 Ah - 3000 Ah and the time is 10 s - 30 s, the Joule heat current is too low, the instantaneous heat release is insufficient, the electrochemical reaction is incomplete, and part of the iron phosphate cannot be converted into lithium iron phosphate, resulting in low capacity of the material. If the Joule heat current is too high, the instantaneous heat release is too large, and lithium iron phosphate will be further converted into iron pyrophosphate and iron phosphide, resulting in the loss of active substances and low capacity of the battery cell.

[0076] The second aspect of the present application provides a small-particle lithium iron phosphate cathode material, which is prepared by the preparation method of the small-particle lithium iron phosphate cathode material described above.

[0077] In some embodiments, the primary particle size D50 of the small-particle lithium iron phosphate cathode material is less than or equal to 200 nm.

[0078] Optionally, the primary particle size D50 of the small-particle lithium iron phosphate cathode material can be 10 nm, 20 nm, 50 nm, 100 nm, 150 nm, 200 nm, or any value less than or equal to 200 nm.

[0079] The third aspect of the present application provides a lithium-ion battery, including the small-particle lithium iron phosphate cathode material described above.

[0080] The fourth aspect of the present application is an electrical device, including the lithium-ion battery described above.

[0081] It should be noted that the electrical equipment can be, but is not limited to, mobile devices, electric vehicles, electric trains, ships, satellites, energy storage systems, etc.; among them, the mobile devices can include, but are not limited to, at least one of mobile phones, laptop computers, etc.; the electric vehicles can include, but are not limited to, at least one of pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.

[0082] The implementation scheme of the present application will be described in detail below in conjunction with specific embodiments. However, those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. For those not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0083] Example 1

[0084] This example provides a lithium iron phosphate cathode material in small particles and its preparation method. The specific steps include:

[0085] 1) Raw material mixing: Add iron phosphate, lithium carbonate, glucose and solvent water to a sand mill according to a mass ratio of 1:0.5:0.2:1.75, and add zirconium balls with a diameter of 0.05 mm for grinding; the grinding time of the sand mill is 1 hour.

[0086] 2) Spray drying: Use a spray dryer to dry the slurry obtained in step 1) to obtain a dried powder material.

[0087] 3) Cold pressing and tablet making: Transfer the sample obtained in step 2) to a tablet press for tablet pressing to obtain a raw tablet. The tablet pressing pressure is 500 MPa. The raw tablet is circular with a diameter of 30 mm and a thickness of 7 mm.

[0088] 4) Heating reaction: In a glove box protected by an inert gas, place the raw tablet sample obtained in step 3) between two pieces of graphite paper in a sandwich shape. The edges of the graphite sheets are connected to wires. Apply a current of 2000 Ah to the graphite sheets through a current-voltage controller for 20 seconds. After natural cooling, take out the sample, which is the lithium iron phosphate cathode material in small particles prepared.

[0089] The SEM of the lithium iron phosphate cathode material in small particles is as Figure 1 shown.

[0090] Example 2

[0091] 1) Raw material mixing: Add iron phosphate, lithium hydroxide, glucose, and solvent water to a sand mill according to a mass ratio of 1:0.4:0.1:1.5, and add zirconium balls with a diameter of 0.01 mm for grinding; the sand milling time of the sand mill is 0.5 hours;

[0092] 2) Spray drying: Use a spray dryer to dry the slurry obtained in step 1) to obtain a dried powder material;

[0093] 3) Cold pressing and tablet forming: Transfer the sample obtained in step 2) to a tablet press for tablet pressing to obtain a raw tablet. The tablet pressing pressure is 300 MPa. The size of the raw tablet is a circle with a diameter of 20 mm, and the thickness of the raw tablet is 5 mm;

[0094] 4) Heating reaction: In a glove box protected by inert gas, place the raw tablet sample obtained in step 3) between two pieces of graphite paper in a sandwich shape. Connect the edges of the graphite sheets to the wires, apply a current of 1000 Ah to the graphite sheets through a current-voltage controller for a time of 10 seconds. After natural cooling, take out the sample, which is the prepared small-particle lithium iron phosphate cathode material.

[0095] Example 3

[0096] 1) Raw material mixing: Add iron phosphate, lithium hydroxide, sucrose, and solvent water to a sand mill according to a mass ratio of 1:0.6:0.3:2, and add zirconium balls with a diameter of 0.2 mm for grinding; the sand milling time of the sand mill is 2 hours;

[0097] 2) Spray drying: Use a spray dryer to dry the slurry obtained in step 1) to obtain a dried powder material;

[0098] 3) Cold pressing and tablet forming: Transfer the sample obtained in step 2) to a tablet press for tablet pressing to obtain a raw tablet. The tablet pressing pressure is 800 MPa. The size of the raw tablet is a circle with a diameter of 40 mm, and the thickness of the raw tablet is 10 mm;

[0099] 4) Heating reaction: In a glove box protected by inert gas, place the raw tablet sample obtained in step 3) between two pieces of graphite paper in a sandwich shape. Connect the edges of the graphite sheets to the wires, apply a current of 3000 Ah to the graphite sheets through a current-voltage controller for a time of 30 seconds. After natural cooling, take out the sample, which is the prepared small-particle lithium iron phosphate cathode material.

[0100] Comparative Example 1

[0101] The difference between this comparative example and Example 1 is that solid-phase method is used for heating. Specifically, the sample obtained after spray drying is subjected to high-temperature heating reaction in a reducing atmosphere to prepare lithium iron phosphate material. Among them, the holding temperature of the high-temperature heating is 780 °C and the time is 20 h.

[0102] The SEM of the lithium iron phosphate material prepared in this comparative example is as Figure 2 shown.

[0103] Comparative Example 2

[0104] The difference between this comparative example and Example 1 is that the applied current for Joule heat treatment is 500 Ah and the time is 5 s.

[0105] Comparative Example 3

[0106] The difference between this comparative example and Example 1 is that the applied current for Joule heat treatment is 4000 Ah and the time is 60 s.

[0107] The lithium iron phosphate cathode materials prepared in the above examples and comparative examples are used as active substances respectively to prepare lithium ion batteries.

[0108] The lithium iron phosphate cathode material is used as the active substance, conductive carbon black is used as the conductive agent, and polyvinylidene fluoride is used as the binder, and they are mixed evenly according to the mass ratio of 9:0.5:0.5. Methylpyrrolidone is used as the solvent to prepare a slurry, which is stirred evenly and coated on aluminum foil. It is dried in a vacuum drying oven at 80 °C for 4 h, and a φ14 mm positive electrode sheet is made by a slicing machine. Using a metal lithium sheet as the counter electrode and 1 mol / L LiPF6 / ethylene carbonate + dimethyl carbonate + ethyl methyl carbonate (volume ratio 1:1:1) as the electrolyte, a CR2032 type button cell is assembled in a glove box under an argon atmosphere.

[0109] The prepared lithium ion batteries are respectively subjected to performance tests. The LAND battery test system CT2001A is used to perform constant current charge and discharge tests on the button cells, and the charge and discharge voltage range is 2.0 - 3.75 V. The test steps are to perform 0.1C / 0.1C charge and discharge cycles for one week first, and then perform 1C / 1C charge and discharge cycles for 5 weeks.

[0110] The specific D50 of the primary particles of the lithium iron phosphate cathode material and the performance test results of the lithium ion batteries are shown in Table 1.

[0111] Table 1 D50 and performance test

[0112]

[0113] Analysis:

[0114] The primary particle size D50 of Example 1 is 295 nm. In Example 2, the size of the sand milling zirconia balls is reduced compared to Example 1, and the grinding particle size of the raw material iron phosphate is lower, resulting in a smaller particle size of the lithium iron phosphate product. In contrast, the primary particle size of the lithium iron phosphate particles prepared by the traditional solid-phase method is much larger than that of this technical solution due to the influence of particle growth during sintering. The larger primary particle size leads to poor rate performance of the material, and the discharge specific capacity at 1C is 3.1 mAh / g lower than that of the material in Example 1. In addition, the results of Comparative Example 2 show that reducing the Joule heating current results in insufficient heat release and incomplete chemical reactions, leading to low capacity of the material, and the discharge specific capacity is 10.4 mAh / g lower than that of the sample in Example 1. At the same time, increasing the Joule heating current and prolonging the reaction time will also cause some particles to sinter and transform into impurities such as iron pyrophosphate and iron phosphide, resulting in a decrease in the initial discharge specific capacity.

[0115] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

[0116] In addition, those skilled in the art can understand that although some embodiments herein include certain features included in other embodiments but not others, the combination of features of different embodiments means that it is within the scope of the present application and forms different embodiments. For example, in the claims above, any one of the claimed embodiments can be used in any combination. The information disclosed in this background art section is only intended to deepen the understanding of the overall background art of the present application and should not be regarded as an admission or any form of implication that this information constitutes prior art already known to those skilled in the art.

Claims

1. A preparation method of a lithium iron phosphate cathode material with small particles, characterized in that, Comprising: Mixing iron phosphate, a lithium source, a carbon source and a solvent to obtain a mixture, and subjecting the mixture to spray drying and tabletting to obtain a raw tablet of the mixture; Under an inert gas, arranging the raw tablet of the mixture between graphite sheets, and applying electric current for Joule heat treatment to obtain a small-particle lithium iron phosphate cathode material.

2. The preparation method of the lithium iron phosphate cathode material of small particles according to claim 1, characterized in that, Satisfying at least one of the following conditions: A. The lithium source includes lithium carbonate and / or lithium hydroxide; B. The carbon source includes glucose and / or sucrose; C. The solvent includes water; D. The mass ratio of the iron phosphate, the lithium source, the carbon source and the solvent is 1:(0.4 - 0.6):(0.1 - 0.3):(1.5 - 2.0).

3. The preparation method of the lithium iron phosphate cathode material of small particles according to claim 1, characterized in that, Before performing the spray drying, the mixture is also ground.

4. The preparation method of the lithium iron phosphate cathode material of small particles according to claim 3, characterized in that, Satisfying at least one of the following conditions: A. The grinding time is 0.5 h - 2 h; B. The particle size of the zirconium balls for grinding is 0.01 mm - 0.2 mm.

5. The preparation method of the lithium iron phosphate cathode material of small particles according to claim 1, characterized in that, Satisfying at least one of the following conditions: A. The pressure for the tabletting is 300 MPa - 800 MPa; B. The diameter of the raw tablet of the mixture is 20 mm - 40 mm; C. The thickness of the raw tablet of the mixture is 5 mm - 10 mm.

6. The preparation method of the lithium iron phosphate cathode material of small particles according to any one of claims 1-5, characterized in that, The applied current for the Joule heat treatment is 1000 Ah - 3000 Ah, and the time is 10 s - 30 s.

7. A lithium iron phosphate cathode material in the form of small particles, characterized in that, Prepared by the preparation method of the small-particle lithium iron phosphate cathode material according to any one of claims 1 - 6.

8. The lithium iron phosphate cathode material for small particles according to claim 7, characterized in that The primary particle size D50 of the small-particle lithium iron phosphate cathode material is less than or equal to 200 nm.

9. A lithium-ion battery, characterized in that, Comprising the small-particle lithium iron phosphate cathode material according to claim 7 or 8.

10. An electrical device, characterized in that, Comprising the lithium ion battery according to claim 9.

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